Issue with updating keySource field

There is an issue with updating source field of relation. While initializing source field is removed and relation field appears. But updating will not remove source field and may cause future problems. I've created 2 test cases to show this issue. One is case when i get problem and second is case that displays a source of a problem.
I'm not so familiar with BR internals yet and I'm not sure with fix. But will propose: in updateRelations to check if keySource != key and typeof attributes[keySource] != 'undefined' then update model and delete attributes[keySource]. Smth like this.